Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. The power section is commonly referred to as the power end by the users and hereafter in this application. The fluid section is commonly referred to as the fluid end by the users and hereafter in this application. Commonly used fluid sections usually comprise a plunger pump housing having a suction valve in a suction bore, a discharge valve in a discharge bore, an access bore, and a plunger in a plunger bore, plus high-pressure seals, retainers, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid end showing its connection to a power end by stay rods. FIG. 1 also illustrates a fluid chamber which is one internal section of the housing containing the valves, seats, plungers, plunger packing, retainers, covers, and miscellaneous seals previously described. A plurality of fluid chambers similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid end housing schematically illustrated in FIG. 2. It is common practice for the centerline of the plunger bore and access bore to be collinear. Typically in the prior art, the centerlines of the plunger bore, discharge bore, suction bore, and access bore are all arranged in a common plane. The spacing of the plunger bores, plungers, plunger packing, and plunger gland nut within each fluid chamber is fixed by the spacing of the crank throws and crank bearings on the crankshaft in the power end of the pump.
Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the valve body, which reversibly seals against the valve seat. In other applications, the term “valve” includes components in addition to the valve body, such as the valve seat and the housing that contains the valve body and valve seat. A valve as described herein comprises a valve body and a corresponding valve seat, the valve body typically incorporating an elastomeric seal within a peripheral seal retention groove.
Valves can be mounted in the fluid end of a high-pressure pump incorporating positive displacement pistons or plungers in multiple cylinders. Such valves typically experience high pressures and repetitive impact loading of the valve body and valve seat. These severe operating conditions have in the past often resulted in leakage and/or premature valve failure due to metal wear and fatigue. In overcoming such failure modes, special attention is focused on valve sealing surfaces (contact areas) where the valve body contacts the valve seat intermittently for reversibly blocking fluid flow through a valve.
Valve sealing surfaces are subject to exceptionally harsh conditions in exploring and drilling for oil and gas, as well as in their production. For example, producers often must resort to “enhanced recovery” methods to insure that an oil well is producing at a rate that is profitable. And one of the most common methods of enhancing recovery from an oil well is known as fracturing. During fracturing, cracks are created in the rock of an oil bearing formation by application of high hydraulic pressure. Immediately following fracturing, a slurry comprising sand and/or other particulate material is pumped into the cracks under high pressure so they will remain propped open after hydraulic pressure is released from the well. With the cracks thus held open, the flow of oil through the rock formation toward the well is usually increased.
The industry term for particulate material in the slurry used to prop open the cracks created by fracturing is the propend. And in cases of very high pressures within a rock formation, the propend may comprise extremely small aluminum oxide spheres instead of sand. Aluminum oxide spheres may be preferred because their spherical shape gives them higher compressive strength than angular sand grains. Such high compressive strength is needed to withstand pressures tending to close cracks that were opened by fracturing. Unfortunately, both sand and aluminum oxide slurries are very abrasive, typically causing rapid wear of many component parts in the positive displacement plunger pumps through which they flow. Accelerated wear is particularly noticeable in plunger seals and in the suction (i.e., intake) and discharge valves of these pumps.
A valve (comprising a valve body and valve seat) that is representative of an example full open design valve and seat for a fracturing plunger pump is schematically illustrated in FIG. 3. FIG. 4 shows how sand and/or aluminum oxide spheres may become trapped between seating surface of valve body and seating surface of valve seat as the suction valve closes during the pump's pressure stroke.
The valve of FIG. 3 is shown in the open position. FIG. 4 shows how accelerated wear begins shortly after the valve starts to close due to back pressure. For valve, back pressure tends to close the valve when downstream pressure exceeds upstream pressure. For example, when valve is used as a suction valve, back pressure is present on the valve during the pump plunger's pressure stroke (i.e., when internal pump pressure becomes higher than the pressure of the intake slurry stream. During each pressure stroke, when the intake slurry stream is thus blocked by a closed suction valve, internal pump pressure rises and slurry is discharged from the pump through a discharge valve. For a discharge valve, back pressure tending to close the valve arises whenever downstream pressure in the slurry stream (which remains relatively high) becomes greater than internal pump pressure (which is briefly reduced each time the pump plunger is withdrawn as more slurry is sucked into the pump through the open suction valve).
When back pressure begins to act on a valve, slurry particles become trapped in the narrow space that still separates the sealing surfaces of the valve body and seat. This trapping occurs because the valve is not fully closed, but the valve body's elastomeric seal has already formed an initial seal against the valve seat. The narrow space shown in FIG. 4 between metallic sealing surfaces of the valve body and valve seat is typically about 0.040 to about 0.080 inches wide; this width (being measured perpendicular to the sealing surfaces of the valve body and seat) is called the standoff distance. The size of the standoff distance is determined by the portion of the valve body's elastomeric seal that protrudes beyond the adjacent valve body sealing surfaces to initially contact, and form a seal against, the valve seat. As schematically illustrated in FIG. 4, establishment of this initial seal by an elastomeric member creates a circular recess or pocket that tends to trap particulate matter in the slurry flowing through the valve.
Formation of an initial seal as a valve is closing under back pressure immediately stops slurry flow through the valve. Swiftly rising back pressure tends to drive slurry backwards through the now-sealed valve, but since back-flow is blocked by the initial valve sealing, pressure builds rapidly on the entire valve body. This pressure acts on the area of the valve body circumscribed by its elastomeric seal to create a large force component tending to completely close the valve. For example, a 5-inch valve exposed to a back pressure of 15,000 pounds per square inch will experience a valve closure force that may exceed 200,000 pounds.
The large valve closure force almost instantaneously drives the affected valve, whether suction or discharge, to the fully closed position where the metal sealing surface of the valve body contacts the corresponding metal sealing surface of the valve seat. As the valve body moves quickly through the standoff distance toward closure with the valve seat, the elastomeric seal insert is compressed, thus forming an even stronger seal around any slurry particles that may have been trapped between the seal insert and the valve seat.
Simultaneously, the large valve closure force acting through the standoff distance generates tremendous impact energy that is released against the slurry particles trapped between the metallic sealing surfaces of the valve body and the valve seat. As shown in FIG. 4, the slurry particles that are trapped between approaching valve seating surfaces are crushed.
In addition to the crushing action described above, slurry particles are also dragged between the valve sealing surfaces in a grinding motion. This grinding action occurs because valve bodies and seats are built with complementary tapers on the sealing surfaces to give the valve a self-alignment feature as the valve body closes against the seat. As the large valve closing force pushes the valve body into closer contact with the seat, the valve body tends to slide down the sealing surface taper by a very small amount. Any crushed slurry particles previously trapped between the sealing surfaces are then ground against these surfaces, resulting in extreme abrasive action.
Extrusion of the elastomeric valve insert seal is also a major factor influencing the performance of valves and seats for high-pressure oil field plunger pumps when pumping sand slurries. FIG. 3 schematically illustrates a cross-section of a typical high-pressure pump valve comprising a valve seat with a corresponding valve body having a peripheral elastomeric valve insert seal. As the valve of FIG. 3 closes, FIG. 4 schematically shows that the elastomeric seal in a peripheral seal retention groove contacts the valve seat while the valve body's impact area is still separated from the valve seat by a gap (identified herein as the extrusion gap). Trapped sand or propend particles from the pumped fluid are schematically illustrated in the extrusion gap in FIG. 4. FIGS. 5A and 5B also show the trapped sand or propend particles in the extrusion gap, as well as a portion of the peripheral elastomeric seal that has extruded into the gap. Extrusion of the peripheral elastomeric seal into the gap typically occurs due to high back pressure acting on the seal. High back pressure also tends to quickly close the valve body against the valve seat, thus closing the gap. But particulate matter in the pumped fluid that is trapped in the gap between the impact area of the valve body and the valve seat may prop the gap open.
During valve closure the elastomeric seal typically seals against the valve seat slightly before contact of the valve body impact area and the valve seat. Extrusion of seal elastomer into the extrusion gap thus begins while the valve body impact area is approaching the valve seat. If the extrusion gap is prevented from completely closing by trapped particulate matter, seal extrusion becomes more severe as back pressure on the valve increases (see FIGS. 5A and 5B). Such extrusion leads to weakening and tearing of the seal elastomer, causing extrusion damage (see FIG. 6), seal failure and premature valve failure.
Typically the motion of the valve body is controlled by valve guide legs attached to the bottom or upstream side of the valve body as shown in FIG. 3. Unfortunately these guide legs are another source of accelerated valve and seat failure when pumping high sand slurry concentrations. FIG. 7A illustrates an old style valve design, circa 1970, in which the valve legs are forged into the upstream side of the valve body; typical of a Mission Service Master I design. FIG. 7B illustrates the slurry flow patterns around the leg; as can be seen in the figure, the downstream side of the legs generates considerable turbulence in the flow. The swirling turbulence in the sand slurry used in typical fracturing work results in sever abrasion of the metal valve body and the elastomeric insert seal, which quickly damages the seal, resulting in seal failure. Once the seal fails on the valve insert, the high pressure fluid on the downstream side of the valve escapes through the seal failure to the low pressure upstream side of the valve. Travelling from the very high pressure to the very low pressure side of the valve results in extreme velocities of the sand slurry, which rapidly erodes the metal valve body and the guide legs in the slurry's path; many times destroying the entire valve leg. Engineers typically recognize the beginning of this failure by four (4) erosion marks behind each leg on worn valves removed from the pump just prior to catastrophic failure in which one or more of the valve legs are completely destroyed by the high pressure erosion. The abraded seal and erosion of the metal valve body are also illustrated in FIG. 7B.
The development of the Roughneck valve design, circa 1983, and later the Mission Service Master II valve or the Novatech valve shown in FIG. 8A greatly improved the flow behind the legs. These designs featured streamlined legs which were achieved by inertia welding an investment guide leg casting to the valve body forging. The streamlined legs and the open area below the valve body and downstream of the guide legs eliminated much of the turbulence behind the guide legs. However in severe pumping environments with high pump rates and high slurry concentrations the problem described in the previous paragraph still existed as evidenced by the four (4) erosion marks and destroyed guide legs. FIG. 8B is a picture a state of the art valve damaged by seal failure and severe erosion behind the guide legs.
The most obvious solution to the problem described above is the removal of the guide legs and somehow guide the motion of the valve by other means. Top stem valves, illustrated in FIGS. 9A, 9B, and 10, attempt this solution. A similar top Stem Valve is illustrated in U.S. Pat. No. 6,698,450. However top stem design valves are inherently unstable in the open position. Once pushed off center by hydraulic flow as illustrated in FIG. 11A, the forces on the valve tend to push the valve further off center. As the valve continues its cyclic repeating opening and closing, the sliding forces cause rapid and accelerating wear on the top stem guide.
The problem of instability is best visualized by comparing a top stem guided valve to the arrow of a weather vane. A weather vane rotates around a pivot point. On a weather vane stabilizing moment is generated by the aerodynamic force on the tail fin multiplied by the moment arm measured from the pivot point to center of the aerodynamic force. This stabilizing force is countered by a destabilizing moment generated by the aerodynamic force on the arrow tip at the front of the vane multiplied by the moment arm measured from the pivot point to center of the force on the arrow. The weather vane always points into the direction of the flow (wind) because there is always more moment or stabilizing force from the tail fins behind the pivot point than moment in front of the pivot point. If the pivot point were moved to the very front or tip of the arrow, then the vane would be extremely stable. Conversely, if the pivot point were moved to the extreme rear of the arrow the vane would be extremely unstable in a fluid flow. Furthermore, if the vane pivots at the rear and as a consequence the vane moves off center to the flow, the vane will move 180 degrees in the flow. Analogizing this to valve in fluid flow, without guidance, the valve would tumble and never close properly.
While weather vanes typically use fins to stabilize the vane, conical or cylindrical surfaces at the rear of the vane would function similarly to fins on a typical weather vane. Valves do not have vanes parallel to flow as do weather vanes; however valves do have conical and cylindrical surfaces with the axis of these surfaces parallel to the flow. The bottom conical valve surface can function to stabilize the valve in fluid flow depending on the location of the pivot point. Compare the bottom surface of a similar body, the Apollo Space Capsules. These capsules have a similar shaped body to a valve without guide legs.
FIG. 10 is a partial cross-section schematically illustrating discharge valve body in its closed position (i.e., with peripheral elastomeric seal held in symmetrical contact with valve seat by discharge valve spring). Note that top guide stem of discharge valve body is aligned in close sliding contact with top valve stem guide.
FIG. 11A illustrates flow around the discharge valve when the valve is in the fully open position, immediately after first opening of the valve. However there are hydrodynamic forces on the valve from the flow which is making a significant change in direction in order to exit through the discharge manifold. These forces quickly move the valve to a position illustrated in FIG. 11B. As shown in FIG. 11B, the valve moves downstream in the flow however the valve's movement is limited by the valve stem guide in which the valve's top stem translates. The valve in turn rotates around the pivot point which is the circumferential edge at the bottom of the valve guide. Because the pivot point is above the area on which the hydrodynamic forces act, the valve is continuously pushed off center. FIG. 11C is the side view of the valve illustrating the area of the valve on which the hydrodynamic forces act, where A1 is the side area of the valve subjected to hydrodynamic forces that would stabilize the valve and A2 is the side area of the valve subjected to forces that would destabilize the valve in fluid flow around the valve. Because the pivot point is above the area on which the hydrodynamic forces act, the valve is continuously pushed off center. The destabilizing moment is equal to the product of the side area of the valve, A2, multiplied by the length of the destabilizing arm or Arm 2 which is measured from the centroid of the area CT2 to the pivot point. As there is no area, A1, above the pivot point, CT1 does not exist, then the stabilizing arm length Arm 1 equals zero and the product of A1 times Arm 1 equals zero; the stabilizing moment then also equals zero. Thus top stem guided valves act similar to a weather vane in which the pivot point is positioned at the rear of the vane. Expressed mathematically:A1×Arm 1=0<A2×Arm 2.
FIG. 11D schematically illustrates how misalignment of top guide stem is possible with excessive wear of top valve stem guide. Such excessive wear can occur because discharge valve body, including top guide stem, is typically made of steel that has been carburized to a hardness of about 60 Rockwell C. In contrast, the wall of top valve stem guide, which is shown in FIG. 11 as being formed within discharge cover, is typically made of mild alloy steel with a hardness of about 30 Rockwell C. Thus the softer wall of stem guide is worn away by sliding contact with the harder guide stem. This wear is accelerated by side loads on valve body that result when fluid flowing past the valve body changes its direction of flow into the discharge manifold. Analogous side loads would be present on a suction valve when fluid flowing past the valve body changes its direction of flow into the plunger cavity.
Eventually, top valve stem guide can be worn sufficiently to allow discharge valve leakage due to significant asymmetric contact of elastomeric seal with valve seat as schematically illustrated in FIG. 12. This problem of stem guide wear is typically addressed in practice through use of a replaceable bushing having a modified top valve stem guide (see the schematic illustration in FIG. 13). Bushing is commonly made of a plastic such as urethane, or a wear and corrosion-resistant metal such as bronze. Such bushings require periodic checking and replacement, but these steps may be overlooked by pump mechanics until a valve fails prematurely.
When the open valve is badly misaligned and the valve guide is badly worn there are not aligning forces available to properly align the valve as it closes. Thus the valve will close against the seat in a miss-aligned or cocked position as shown in FIG. 12. In this position, the side of the cocked valve leaves an extrusion gap that results in shorten valve insert seal life. The cocked valve also results in uneven loading of the metal valve body against the seating surface of the seat resulting in accelerated metal wear on the valve body and seat.